Among the fallout is the question of whether United could have found a way to resolve the situation without help from security.

Here's what happened: United needed to make room on its full flight from Chicago to Louisville, Kentucky, for four crew members who were needed at the plane's final destination. The airline says it asked volunteers to give up their seats in exchange for as much as $1,000, but when no one volunteered the gate agent selected four people to exit the plane.

A 69-year-old male passenger, who has been identified as David Dao, a doctor living in Kentucky, refused to give up his seat after being told by United employees he must leave. Three police officers working for the Chicago Aviation Department then forcibly removed Dao from the plane.

United referred to its " Contract of Carriage" in this situation, which says it can deny boarding to passengers if a flight is overbooked and no passengers volunteer to give up their seat. ( United says boarding refers to any period before the flight physically takes off, even if passengers are already on the plane.)

It's unclear whether a higher amount would have resolved Sunday's incident differently, but United does not mention any cap on how much it can offer volunteers to give up their seats in its contract:

"UA will request Passengers who are willing to relinquish their confirmed reserved space in exchange for compensation in an amount determined by UA (including but not limited to check or an electronic travel certificate)."

Additionally, there's no federal limit on how much a carrier can offer volunteers, as written in the Department of Transportation's " Fly Rights":

"DOT has not mandated the form or amount of compensation that airlines offer to volunteers. DOT does, however, require airlines to advise any volunteer whether he or she might be involuntarily bumped and, if that were to occur, the amount of compensation that would be due. Carriers can negotiate with their passengers for mutually acceptable compensation."

The Department of Transportation limits the amount an airline can compensate passengers who are involuntarily bumped from a plane to a maximum of $1,350. There is no federal limit, however, on the amount an airline can offer volunteers to give up their seat.

In an interview with Business Insider, aviation analyst Henry Harteveldt questioned why United didn't try to offer a bigger incentive when the situation started to escalate.

"There's no limit to what an airline can pay," Harteveldt said. "It's generally cheaper for an airline to proactively ask people to give up their seats because the compensation is usually less."

Harteveldt added that it was in the best interest of an airline to offer bigger incentives because the carrier doesn't need to disclose when people volunteer to give up their seats. An airline must tell the Department of Transportation whenever it denies boarding to a passenger.

A Delta customer wrote in a Forbes article that her family once received $1,350 a ticket for volunteering to give up her seat on a flight from New York to Florida.

Getty Images/Spencer Platt

United's contract, which customers agree to when they buy a ticket, says the airline will select who will be denied boarding based on the "passenger's fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment."

Passengers with disabilities and people under the age of 18 will be the last to be denied boarding, according to United.

But there are also questions as to why United began looking for passengers to give up their seats once they were already on the plane. As mentioned earlier, United says it can invoke its involuntary boarding policy at any point before the plane takes off.

But Harteveldt described this as highly unorthodox.

"I cannot recall the last time I have seen or heard about a gate agent going onto a plane to remove a revenue customer from that flight because of involuntary denied boarding," Harteveldt said. "To remove a paying customer from a flight is extremely rare."